The present invention relates to an aluminum titanate ceramic and a process for producing said aluminum titanate ceramic. More particularly, the present invention relates to an aluminum titanate ceramic used in, for example, a head port liner, an exhaust manifold liner (these liners are for the heat insulation of the inside of an engine exhaust pipe) and a catalytic converter, as well as to a process for producing said aluminum titanate ceramic.
A ceramic using aluminum titanate as a base material has a low thermal expansion coefficient and a small Young's modulus. Hence, it is suitable for use in various members used under severe conditions in which the members are required to have high thermal shock resistance and low thermal expansion, for example, in a head port liner, an exhaust manifold liner (these liners are for the heat insulation of the inside of gasoline engine exhaust pipe) and in a catalytic converter.
It is known that in ordinary aluminum titanate materials (aluminum titanate material is hereinafter referred to as AT material), the Young's modulus and the strength are generally proportional to each other. That is, as the strength is higher, the Young's modulus is larger. Conversely, as the strength is lower, the Young's modulus is smaller. This is because when an AT material has a low strength, the crystal grains of the AT material are generally large and a number of cracks exist between the grain boundaries, allowing the AT material to be easily deflected and lowering its strength. Many of the conventional AT materials have a Young's modulus of about 2,000 Kgf/mm.sup.2 or more and a bending strength of 2-5 Kgf/mm.sup.2.
Various improvements have been made for AT materials, depending upon their applications, by adding additives, etc. thereto. In this connection, it was proposed to add a rare earth oxide in order to suppress the decomposition of aluminum titanate at high temperatures and impart a high strength.
For example, Japanese Patent Publication No.3629/1982 proposes a low-thermal-expansion ceramic comprising, as a main phase, an aluminum titanate containing at least one rare earth element selected from the group consisting of Y, La and Ce. Japanese Patent Application Kokai (Laid-Open) No. 257165/1989 proposes an aluminum titanate containing a rare earth oxide, mullite and iron titanate, which is stable even at high temperatures of about 1,000.degree.-1,300.degree. C. Japanese Patent Application Kokai (Laid-Open) No. 258670/1990 proposes a low-thermal-expansion ceramic consisting of an aluminum titanate-magnesium titanate solid solution and Yttrium titanate, which is stable at high temperatures.
The ceramics proposed in Japanese Patent Publication No. 3629/1982 and Japanese Patent Application Kokai (Laid-Open) No. 258670/1990, contain rare earth element(s). However, they have a small number of cracks and consequently have a high strength and a high Young's modulus; when used as a casting material for metal they give low deflection and are unable to absorb the strain applied, that is, they are inferior in casting; when subjected to a heat cycle wherein high temperature heating and cooling are repeated, new cracks appear at the grain boundaries and thereby their strengths are deteriorated.
The aluminum titanate proposed in Japanese Patent Application Kokai (Laid-Open) No. 257165/1989 contains mullite and a rare earth oxide. However, it is inferior in casting similarly to the above ceramics, and further has insufficient heat cycle durability because it has cracks in the crystal grains and at the grain boundaries.